Resin compound, and adhesive film, metal-clad adhesive film, circuit board, and assembly structure, using the resin compound

ABSTRACT

The present invention provides a resin compound, which causes very little thermal stress even when a flip chip is mounted, is of low elastic modulus, and has high heat resistance enough for solder floating at 300° C., and also an adhesive film, metal-clad adhesive film, circuit board, and assembly structure using the resin compound.  
     The present invention is a resin compound containing (A) polyamide imide with siloxane bond, (B) acrylic polymer with weight-average molecular weight of 500,000 or more, (C) thermoset resin, and (D) solvent; an adhesive film and a metal-clad adhesive film using the resin compound; and also a circuit board and an assembly structure made with these films.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a resin compound and an adhesive film with excellent heat resistance, electrical properties, adhesion to metal and at the same time low elastic modulus and also to a circuit board and an assembly structure using these. The resin compound and adhesive film of the invention are applicable widely to the surface circuit layer and/or stress compliance layer of a circuit board. Because the circuit board of the invention is of low elastic modulus, it is most suitable for flip chip technology.

[0002] Latest technical innovation is remarkable in making the electronic devices such as personal mobile data terminals smaller in size, lighter in weight, and higher in performance. As an assembly structure of these electronic devices, a flip chip technology or chip scale package (CSP) structure that enables to realize high-density and compact packaging has come to be employed instead of a conventional resin encapsulated semiconductor package using a resin compound made of epoxy resin and inorganic filler. As above, a high-density compact package is frequently made of a grid array structure because, when the package is mounted on a circuit board, a number of pins must be arranged in a small area as external terminals for the package. Since there is a technical limitation of high-density packaging on a conventional circuit board for the above reason, a board with a surface laminate wiring circuit that has no through-hole but comprises via-holes only has been in use. As a material for forming a fine wiring circuit on its surface, there are available a photo-via material, on which a wiring circuit is formed by exposure and development processes, and a laser-via material, on which holes are machined by laser beam or the like. Brief technology of the above has been disclosed, for example, in “Printed Circuit Board Applicable to High-Density Packaging—Present Status and Problems of Build-up Circuit Board Technology” of “Electronic Material” issued by Industrial Research Association, October 1998 issue.

[0003] Generally speaking, a material with high heat resistance, typically such as polyimide, has high elastic modulus. For this reason, if a packaging device with low coefficient of thermal expansion and high elastic modulus, such as a flip chip, is mounted on a polyimide circuit board, there lies a question in its reliability because of possible disconnection due to the thermal stress caused between the board and the device.

[0004] On the other hand, because heat resistance of a rubber-added material with low elastic modulus is generally lower, there lies a question in the heat resistance against high-temperature bath solder (lead free solder) at about 300° C.

[0005] The present invention provides a resin compound, which causes very little thermal stress even when a flip chip is mounted, is of low elastic modulus, and has high heat resistance enough for solder floating at 300° C., and also an adhesive film, circuit board, and assembly structure using the resin compound.

SUMMARY OOF THE INVENTION

[0006] The means of solving the above problems are as follows.

[0007] The first aspect of the invention is a resin compound containing (A) polyamide imide with siloxane bond, (B) acrylic polymer with weight-average molecular weight of 500,000 or more, (C) thermoset resin, and (D) solvent.

[0008] The second aspect of the invention is an adhesive film having the above-mentioned resin compound. This adhesive film provides an adhesive material that has both low elastic modulus and high heat resistance and also excellent adhesion to metal and electrical properties.

[0009] The use of (A) polyamide imide with siloxane bond as an essential component in the invention makes it possible to maintain low elastic modulus without lowering heat resistance. The polyamide imide with siloxane bond has superior solubility to solvent, as compared to the polyimide having similar heat resistance, and also excellent miscibility with other polymers. It is a suitable component for realizing the effect of the resin compound of the present invention.

[0010] In addition, since the component (A) above has siloxane bond, it has higher miscibility with other components of the invention, including acrylic polymer and epoxy resin. As a result, a uniform film with no layer separation can be provided in a broad range of composition (mixture).

[0011] Particularly in combining with high molecular-weight acrylic polymer, miscibility is an important issue. The siloxane bond content in the acrylic polymer structure is preferred to be 20 to 80 weight %. If it is less than 20 weight %, the miscibility decreases and a uniform resin compound with acrylic polymer cannot be obtained. If it exceeds 80 weight %, the adhesion to metal decreases and, when a fine wiring circuit is formed, results in lower reliability.

[0012] The present invention, causing polyamide imide to contain siloxane bond, can realize excellent miscibility with high molecular-weight acrylic polymer and, at the same time, drastically lower elastic modulus than that of conventional polyamide imide.

[0013] In the invention, (A) polyamide imide with siloxane bond represents a condition where, for example, amide bond or imide bond is bonded with siloxane bond.

[0014] To be concrete, a polymer that at least contains an amide bond, an amide bond as the main chain of an imide precursor, and the following bond unit is preferred:

—NH—CO—X-Z- or —NH—CO—X—CO—NH—Y-Z—

[0015] (X and Y in the above formula are an organic group including an aliphatic or aromatic group, and can be equal or different. Z is siloxane bond.)

[0016] Besides, a polymer that contains a siloxane bond as the end group of an amide bond and an amide bond as the main chain of an imide precursor as follows, for example, is also effective:

—NH—CO—X-Z or —NH—CO—X—CO—NH—Y-Z

[0017] (X and Y in the above formula are an organic group including an aliphatic or aromatic group, and can be equal or different. Z is siloxane bond.)

[0018] The present invention, using (B) acrylic polymer with weight-average molecular weight of 500,000 or more, enables to realize low elastic modulus without lowering heat resistance. If acrylic polymer with weight-average molecular weight of less than 500,000 is used, sufficient properties including heat resistance and electrical properties cannot be attained. The use of high molecular-weight polymer with weight-average molecular weight of 500,000 or more enables to attain stable properties.

[0019] The mix ratio of the above component (A) to component (B) is preferably 3:1 to 1:5 by weight. In mixing the two, from a view point of lowering the elastic modulus, increasing the mix ratio of the component (B), i.e. acrylic polymer is preferable so as to increase the resolution in film forming and the adhesion to metal. However, increasing the component (B), i.e. acrylic polymer in excess of the mix ratio 1:5 results in insufficient properties including heat resistance and electrical properties.

[0020] It is further preferable that the glass transition temperature of the acrylic polymer is 0° C. or less. Because of this, the elastic modulus under a range of 0 to 150° C., a normal operating environment, can be maintained low.

[0021] Since use of a combination of the above components (A) and (B) in the present invention is not sufficient enough to maintain the properties at high temperature, it is preferred to utilize a component that forms a cross-linked structure so as to maintain the solder heat resistance at 300° C. or above.

[0022] Thermoset resin is a component that forms a cross-linked structure. It is available in the form of, for example, epoxy resin, maleimide resin, cyanate resin, isocyanate resin, phenol resin, allyl resin, unsaturated polyester resin, butadiene resin, silicone resin, vinyl ester resin, and silicone resin. Among all, such epoxy resin that comprises an epoxy compound, hardener containing a phenol group, and imidazole curing accelerator is the most preferable.

[0023] Because epoxy resin before cross-linking is generally a low molecular-weight compound or low molecular weight polymer, it contributes to improved wettability in bonding a film with various materials and brings an effect of increasing the adhesion to metal.

[0024] In the present invention, the addition of the thermoset resin component (C) is preferred to be 5 to 50 weight % of the entire film. If it is less than 5 weight %, the effect of utilizing a cross-linked structure formed by the thermoset resin component cannot be produced sufficiently, and consequently sufficient heat resistance at high temperature cannot be attained. If it exceeds 50 weight %, the elastic modulus tends to increase, and consequently the resolution in film forming is negatively affected.

[0025] The epoxy resin of the invention can be a compound used in a semiconductor sealing material or printed circuit board material. It is available in the form of, for example, phenol novolac type epoxy resin, cresol novolac type epoxy resin, bisphenol type epoxy resin including bisphenol A, bisphenol F and bisphenol S type epoxy resin, multi-functional epoxy resin of phenol resin or cresol resin, and bi-functional or tri-functional epoxy resin having a naphthalene skeleton, antracene skeleton or dicyclopentadiene skeleton.

[0026] A publicly known compound is applicable as the hardener for the above epoxy resin. Among all, a compound having two or more phenolic hydroxyl groups is preferable. It is available in the form of, for example, phenol novolac resin, condensation polymer of phenol and aralkyl ether, bisphenol resin, o-cresol novolac resin, and poly p-vinyl phenol.

[0027] As needed, a publicly known compound is applicable as the curing accelerator. It is available in the form of, for example, imidazole compound, tertiary phosphate compound, quaternary phosphate compound, complex of tertiary phosphate compound and borate compound, complex of quaternary phosphate compound and borate compound, tertiary amine, borate complex salt, Lewis acid, inorganic acid, short-chain amide, dihydrazide, titanic ester, and cationic or anionic catalyst. Among all, imidazole compound is preferable because it excellently adds to heat resistance, adhesion, and shelf life of material.

[0028] If the physical properties of the adhesive film of the present invention after heating and curing allow the elastic modulus at 50° C. to be less than 1 GPa, thermal stress caused by temperature variation can be reduced drastically at the time when a flip chip is mounted on the film, and reliability under a thermal cycle and other conditions can be improved tremendously.

[0029] In the present invention, the solvent component (D) has no particular limit provided that the thermoset polymer as the above components (A) and (B) and the epoxy resin, hardener, and curing accelerator as the component (C) can be evenly mixed in it. Preferable solvent is, for example, acetone, methyl ethyl ketone, methyl isobutyl ketone, methyl cellosolve, γ-butyrolactone, N,N-dimethyl formamide, N-methyl-2-pyrrolidone, toluene, dimethyl acetamide, cyclohexanone, or isopropyl alcohol, and a mixed solvent of two or more of the above is allowable.

[0030] An additive such as inorganic filler, colorant, or flame retardant can be added so far as the properties of the resin compound and adhesive film of the invention are not affected.

[0031] The third aspect of the invention provides a metal-clad adhesive film that utilizes the resin compound of the first aspect of the invention as an insulation layer. The use of the metal-clad adhesive film as wiring material for the below-mentioned build-up structure, the process can be simplified.

[0032] The resin compound of the present invention is bonded onto a metal foil by a process that a specified amount of the resin compound containing solvent is applied on a metal foil and then the metal foil and the resin film are bonded together as the solvent is removed by heating, depressurizing, or other processing. The metal foil has no specific limit provided that it serves as a conductor forming a circuit, but copper is preferable. A metal-clad adhesive film is available in a structure where a metal foil is formed on one side of the film or on both sides of the film.

[0033] The fourth aspect of the present invention provides a circuit board of a build-up structure where there is arranged the adhesive film of the second aspect of the invention on the circuit board and the circuit layer of the circuit board is electrically connected to external terminals. The adhesive film to be arranged on the circuit board can be either a simple adhesive film or the above-mentioned metal-clad adhesive film.

[0034] If a simple adhesive film is arranged on a circuit board, the circuit board of a build-up structure can be obtained by a process that, after holes for via-hole are drilled at specified positions using a laser machine or the like, the board is subjected to roughing and conductor plating so as to form a wiring circuit and attain the continuity to the lower circuit board by means of connection through the via-holes. By repeating this process as needed, a build-up circuit board having a multi-layer structure can be provided.

[0035] If a metal-clad adhesive film is arranged on a circuit board, holes are made at necessary positions of the metal foil beforehand by etching or other means and then holes for via-hole are drilled in the film by a laser machine or the like, utilizing the metal foil as a mask. Then, the holes for via-hole are made conductive by plating, and lastly a wiring circuit is etched on the metal foil. Thus, a circuit board having a build-up structure is formed. By repeating this process as needed, a build-up circuit board having a multi-layer structure can be provided.

[0036] The fifth aspect of the present invention is an assembly structure in which a flip chip is mounted on the circuit board of a build-up structure of the fourth aspect of the invention. The surface of a circuit board of a build-up structure to mount a flip chip is made of an insulation layer with low elastic modulus. The use of the assembly structure can drastically reduce the thermal stress to be caused by the difference in the coefficient of thermal expansion between the flip chip and the circuit board. Thus, it becomes possible to secure connection reliability under a thermal cycle and other conditions without using an underfill resin that was needed in a prior art.

[0037] The present invention can reduce cost, including material and process cost, tremendously. Because no underfill resin is needed, the invention allows to perform repair more easily than before and can contribute greatly to the improvement of yield.

BRIEF DESCRIPTION OF DRAWINGS

[0038]FIG. 1 is a Cross-sectional view of an adhesive film.

[0039]FIG. 2 is a Cross-sectional view of an adhesive film on one side of which a metal foil is arranged.

[0040]FIG. 3 is a Cross-sectional view of a circuit board of a build-up structure on which an adhesive film is arranged.

[0041]FIG. 4 is a Cross-sectional view of a circuit board of a build-up structure on which layers of adhesive films are arranged in a multi-layer structure.

[0042]FIG. 5 is a Cross-sectional view of a circuit board of a build-up structure on both sides of which an adhesive film is arranged.

[0043]FIG. 6 is a Cross-sectional view of an assembly structure in which a flip chip is mounted on a circuit board of a build-up structure.

[0044]FIG. 7 is a Cross-sectional view of an adhesive film on both sides of which a metal foil is arranged.

DETAILED DESCRIPTION OF THE INVENTION

[0045] The present invention is explained concretely hereunder, using embodiments.

[0046] [Embodiment 1]

[0047] 100 g of polyamide imide with siloxane (made by Hitachi Chemical Co., Ltd.: PS6800, siloxane content: 49 weight %) and 400 g of high molecular-weight acrylic polymer (made by Hitachi Chemical Co., Ltd.: PS4000, weight-average molecular weight: 900,000) were evenly mixed into a mixed solvent of methyl isobutyl ketone and γ-butyrolactone (weight ratio=2:1).

[0048] In addition, to use maleimide resin as the thermoset resin, 58 g of 4,4-bismaleimide diphenyl methane (made by Mitsui Chemicals, Inc.) and 67 g of 2,2-bis(4-(4-aminophenoxy)phenyl)propane (made by Mitsui Chemicals, Inc.) were added as the components and varnish having a solid content of 25 weight % was produced.

[0049] The varnish was applied over a polyester film, and then the solvent was removed by heating at 130° C. for 5 minutes and a sample of 50 μm thick was produced. The sample was subjected to heating at 200° C. for 30 minutes and finally a hardened product was obtained. The hardened product was then subjected to periodic strain by a visco-elastic measuring apparatus and the complex elastic modulus was measured. The measure storage modulus at 50° C. was 9.6 MPa, and it was confirmed that the product had low elastic modulus (0.1 GPa or less). Besides, the thermal gravimetric measurement of the hardened product at 350° C. resulted in 2% or less, and it was confirmed that the product had excellent heat resistance.

[0050] [Embodiment 2]

[0051] 100 g of polyamide imide with siloxane (made by Hitachi Chemical Co., Ltd.: PS6800, siloxane content: 49 weight %) and 200 g of high molecular-weight acrylic polymer (made by Hitachi Chemical Co., Ltd.: PS4000, weight-average molecular weight: 900,000) were evenly mixed into a mixed solvent of methyl ethyl ketone and γ-butyrolactone (weight ratio=3:1).

[0052] In addition, to use epoxy resin as the thermoset resin, 49 g of cresol novolac type epoxy resin (made by Sumitomo Chemical Co., Ltd.: EOCN 195-3) and 26 g of phenol novolac resin (made by Meiwa Chemical Co., Ltd.: H-1) were added as the components and 1.0 g of tertiary amine compound (made by Sanyo Chemical Co., Ltd.: 1,8-bicyclo(5,4,0)-undecen (DBU) as a curing accelerator, and varnish having a solid content of 20 weight % was produced.

[0053] The varnish was applied over a polyester film, and then the solvent was removed by heating at 120° C. for 10 minutes and a sample of 70 μm thick was produced. The sample was subjected to heating at 200° C. for 30 minutes and finally a hardened product was obtained. The storage modulus of the hardened product at 50° C. was measured in a similar manner as in Embodiment 1. The result was 15.3 MPa, and it was confirmed that the product had low elastic modulus (0.1 GPa or less). Besides, the thermal gravimetric measurement of the hardened product at 350° C. resulted in 1% or less, and it was confirmed that the product had excellent heat resistance. Further, when the hardened product was made to absorb moisture in a PCT (pressure cooker test) at 121° C. for 168 hours, the coefficient of saturated moisture absorption was 0.85 weight %, which means the use of epoxy compound as a cross-linking component results in lower coefficient of saturated moisture absorption (2 weight % or less).

[0054] [Embodiment 3]

[0055] 200 g of polyamide imide with siloxane (made by Hitachi Chemical Co., Ltd.: PS6800, siloxane content: 49 weight %) and 100 g of high molecular-weight acrylic polymer (experimental product, weight-average molecular weight: 700,000, glass transition temperature: 5° C.) were evenly mixed into a dimethyl formamide solvent.

[0056] In addition, to use cyanate resin as the thermoset resin, 200 g of bi-functional type cyanate resin (made by ASAHI-CHIBA Co., Ltd.: AroCy™M-50) and 10 g of phenol novolac resin (made by Meiwa Chemical Co., Ltd.: H-1) were added as the components and 0.1 g of cobalt naphthanate (made by Wako Pure Chemical Industries, Ltd.), and varnish having a solid content of 30 weight % was produced. Then, the varnish was applied over a polyester film, and then the solvent was removed by heating at 130° C. for 15 minutes and an adhesive film of 80 μm thick as shown in FIG. 1 was produced. Table 1 shows the physical properties of the produced adhesive film. With the adhesive film according to this embodiment, it is possible to attain superior properties that the elastic modulus at 50° C. is low (0.1 GPa or less) and the solder heat resistance is high enough to prevent blister and other faults in solder floating at 300° C.

[0057] [Embodiment 4]

[0058] 100 g of polyamide imide with siloxane (made by Hitachi Chemical Co., Ltd.: PS6500, siloxane content: 35 weight %) and 100 g of high molecular-weight acrylic polymer (experimental product, weight-average molecular weight: 1,200,000, glass transition temperature: 10° C.) were evenly mixed into a cyclohexanone solvent.

[0059] In addition, to use epoxy resin as the thermoset resin, 53 g of multi-functional type epoxy resin (made by Petrochemical Shell Epoxy Co., Ltd.: EP-1032) and 33 g of phenol novolac resin (made by Meiwa Chemical Co., Ltd.: H-1) were added as the components and 0.34 g of imidazole compound (made by Shikoku Chemicals Corp.: 2E4MZ) as a curing accelerator, and varnish having a solid content of 40 weight % was produced.

[0060] The varnish was applied over a polyester film, and then the solvent was removed by heating at 140° C. for 5 minutes and an adhesive film of 60 μm thick as shown in FIG. 1 was produced. Table 1 shows the physical properties of the produced adhesive film. With the adhesive film according to this embodiment, it is possible to attain superior properties that the elastic modulus at 50° C. is low (0.1 GPa or less) and the solder heat resistance is high enough to prevent blister and other faults in solder floating at 300° C. It is also possible to attain an adhesive film with extremely excellent adhesion to copper.

[0061] [Embodiment 5]

[0062] 100 g of polyamide imide with siloxane (made by Hitachi Chemical Co., Ltd.: PS6500, siloxane content: 35 weight %) and 300 g of high molecular-weight acrylic polymer (experimental product, weight-average molecular weight: 800,000, glass transition temperature: −20° C.) were evenly mixed into a N-methyl-2-pyrrolidone solvent.

[0063] In addition, to use maleimide resin as the thermoset resin, 35 g of 2,2-bis(4-(4-maleimide phenoxy)phenyl)propane (made by Mitsui Chemicals, Inc.) and 9 g of 2,2-bis (4-(4-cyanamide phenoxy)phenyl)propane (made by Mitsui Chemicals, Inc.) were added as the components and varnish having a solid content of 50 weight % was produced.

[0064] The varnish was applied over a polyester film, and then the solvent was removed by heating at 125° C. for 20 minutes and an adhesive film of 40 μm thick as shown in FIG. 1 was produced. Table 1 shows the physical properties of the produced adhesive film. By keeping the glass transition temperature of the acrylic polymer at 0° C. or below, the produced film can have low elastic modulus (0.1 GPa or less) in the entire temperature range from 0 to 150° C., a typical operating environment. Lowering the glass transition temperature makes it possible to attain higher adhesion to copper under room temperature.

[0065] [Embodiment 6]

[0066] 100 g of polyamide imide with siloxane (made by Hitachi Chemical Co., Ltd.: PS6800, siloxane content: 49 weight %) and 300 g of high molecular-weight acrylic polymer (made by Hitachi Chemical Co., Ltd.: PS4000, weight-average molecular weight: 900,000) were evenly mixed into a mixed solvent of methyl isobutyl ketone and cyclohexanone (weight ratio=1:1).

[0067] In addition, to use epoxy resin as the thermoset resin, 65 g of cresol novolac type epoxy resin (made by Sumitomo Chemical Co., Ltd.: EOCN 195-3) and 35 g of phenol novolac resin (made by Meiwa Chemical Co., Ltd.: H-1) were added as the components and 0.45 g of imidazole compound (made by Shikoku Chemicals Corp.: 2E4MZ) as a curing accelerator, and varnish having a solid content of 30 weight % was produced.

[0068] The varnish was applied over a copper foil (0.018 mm thick), and then the solvent was removed by heating at 130° C. for 10 minutes and a metal-clad adhesive film of 50 μm thick as shown in FIG. 2 was produced. Table 1 shows the physical properties of the produced adhesive film. The elastic modulus and glass transition temperature are the properties of the resin only. As explained in this embodiment, a metal-clad adhesive film can be obtained by applying varnish over a metal foil. The adhesion of the resin to copper foil was measured in accordance with the method in JIS C 6481. The measurement showed that the adhesive strength to copper foil was as high as 2.4 kN/m as a result of having applied varnish over a copper foil, as compared to a result of bonding an adhesive film onto a copper foil. In addition, a similar test was conducted using an aluminum foil, and the result was as high as 3.5 kN/m. That is, a metal-clad adhesive film with excellent adhesion to copper, aluminum, 42-alloy, nickel, and other metals was obtained by varying a combination of the epoxy resin.

[0069] [Embodiment 7]

[0070] 90 g of polyamide imide with siloxane (made by Hitachi Chemical Co., Ltd.: PS6800, siloxane content: 49 weight %) and 360 g of high molecular-weight acrylic polymer (experimental product, weight-average molecular weight: 800,000, glass transition temperature: −15° C.) were evenly mixed into a N-methyl-2-pyrrolidone solvent.

[0071] In addition, to use epoxy resin as the thermoset resin, 80 g of biphenyl type epoxy resin (made by Petrochemical Shell Epoxy Co., Ltd.: YX-4000H) and 70 g of xylylene type phenol resin (made by Mitsui Chemicals, Inc.: XL-225) were added as the components and 1.07 g of triphenyl phosphine (made by Hokko Chemical Industry Co., Ltd.: TPP) as a curing accelerator, and varnish having a solid content of 35 weight % was produced.

[0072] The varnish was applied over a copper foil (0.018 mm thick), and then the solvent was removed by heating at 160° C. for 5 minutes and a metal-clad adhesive film of 20 μm thick as shown in FIG. 2 was produced. Similarly to Embodiment 6, Table 1 shows the physical properties of the produced metal-clad adhesive film. Similarly to Embodiment 6, the adhesion of the film to copper foil is extremely high and, additionally, lowering the glass transition temperature of the acrylic polymer makes it possible to attain a metal-clad adhesive tape with far excellent adhesion to copper under room temperature.

[0073] [Embodiment 8]

[0074] The film produced in Embodiment 6 was incorporated, as an insulation layer for forming the surface circuit, on a FR-4 board (ANSI Standard: American National Standard Institute) with 6 layers, and a circuit board of a build-up structure that was electrically connected to the FR-4 board was produced as shown in FIG. 3. Electrodes for measuring the insulation resistance of the surface circuit were provided and the temperature dependency of the insulation resistance was measured.

[0075] The result was 10¹⁵ Ωcm or more under room temperature and 10¹¹ Ωcm at 150° C. The electrodes were made in accordance with the surface resistance measurement method in JIS C 6481. As explained in this embodiment, the film also has excellent electrical properties in the entire temperature range under a normal operating environment.

[0076] A thermal cycle test (−55° C./125° C., 500 cycles) of a circuit board alone was conducted and the result showed that the surface circuit was not affected by stress and consequently no problem such as disconnection was caused because the elastic modulus of the surface insulation layer was low. No separation was observed between the FR-4 board and the surface circuit layer.

[0077]FIG. 4 shows a build-up circuit board with a multi-surface circuit layer structure, produced in a similar manner as in this embodiment. FIG. 5 shows a build-up circuit board, wherein the surface circuit layers produced in a similar manner as in this embodiment are arranged on both sides of the board.

[0078] [Embodiment 9]

[0079] The metal-clad adhesive film produced in Embodiment 7 was incorporated, as an insulation layer for forming the surface circuit, on a FR-4 board (ANSI Standard: American National Standard Institute) with 6 layers, and a circuit board of a build-up structure that was electrically connected to the FR-4 board was produced.

[0080] Ten flip chips of 10 mm×5 mm (bump pitch: 0.15 mm, bump height: 0.1 mm) were mounted on the circuit board as shown in FIG. 6. Then, the board was subjected to a thermal cycle test (−55° C./125° C., 500 cycles) to check the reliability of an assembly and the connection reliability was evaluated by electrical check and visual check. (In FIG. 6., only one flip chip is shown as mounted.) As a result, even without having filled the connected portions with underfill resin, neither electrical disconnection nor mechanical cracking of the bump was observed. Providing an insulation layer of lower elastic modulus on the surface as above makes it possible to attain excellent connection reliability in a thermal cycle test as well as in the temperature range of an operating environment.

[0081] [Comparison Sample 1]

[0082] An adhesive film was produced using the resin compound according to Embodiment 2 except for polyamide imide with siloxane, and the properties were examined.

[0083] [Comparison Sample 2]

[0084] An adhesive film was produced using the resin compound according to Embodiment 2 except for acrylic polymer, and the properties were examined.

[0085] [Comparison Sample 3]

[0086] A build-up circuit board was produced, using the same resin compound as in Comparison Sample 2 and in the same manner as in Embodiment 8, and was evaluated. As a result, separation was caused between the FR-4 board and the surface circuit layer in the thermal cycle test. The separation was caused probably because the elastic modulus of the surface circuit layer was high and consequently stress was caused by the difference in the coefficient of thermal expansion between the FR-4 board and the surface circuit layer.

[0087] [Comparison Sample 4]

[0088] A build-up circuit board using the same resin compound as in Comparison Sample 2 was produced and flip chips were mounted on it in the same manner as in Embodiment 9, and the connection reliability of the board was evaluated. As a result, when no underfill resin was applied, electrical disconnection was caused between the bump and the assembled board in 10 cycles. TABLE 1 Elastic Soldering Adhesive Glass modulus heat strength to transition @50° C. resistance* copper temperature (Mpa) @300° C./2 min. (kN/m) (° C.) Embodiment 3 23.5 OK 0.90 10 Embodiment 4 17.5 OK 1.5 10 Embodiment 5 13.5 OK 1.2 −20 Embodiment 6 12.8 OK 2.4 25 Embodiment 7 8.5 OK 2.7 −15 Comparison 150 NG 0.65 40 Sample 1 Comparison 2000 OK 0.80 200 Sample 2

[0089] Sample: To measure the elastic modulus and glass transition temperature, a film that was hardened at 200° C. for 30 minutes was used. To measure the solder heat resistance, an adhesive film shown in FIG. 7 was used, where a copper foil was arranged on both sides of the film, or on one side in case of a one-sided copper-clad adhesive film, and the film was laminate-bonded at 200° C. for 30 minutes under 2 MPa pressure. To measure the adhesive strength, an adhesive film on which copper was formed by spatter deposition and then a copper layer of 20 μm thick was formed by electro-plating using copper sulfate. The plating condition was 0.5A/dm² for 5 minutes+1.5A/dm² for 40 minutes under room temperature. To determine the adhesive strength of a metal-clad adhesive film, peeling strength of the boundary between the copper foil and the varnish applied was measured.

[0090] Measuring method: To determine the elastic modulus and glass transition temperature, a sample was subjected to periodic strain by a visco-elastic measuring apparatus and the complex elastic modulus was measured. The storage modulus at 50° C. was determined to be the elastic modulus, and the peak temperature of tan δ obtained by the ratio of a real number part to an imaginary number part was determined to be the glass transition temperature.

[0091] The solder heat resistance and peeling strength were measured in accordance with the measurement method in JIS C 6481.

[0092] The resin compound and adhesive film realized by the present invention have both low elastic modulus and high heat resistance and also excellent adhesion to metal and electrical properties. Because of this, the resin compound and adhesive film exhibit extremely superior properties as an insulation layer of a circuit board of a build-up structure on which flip chips and chip scale packages are mounted. In particular, high reliability can be attained without using an underfill structure which is normally employed when flip chips are mounted on a board. As a result, an assembly structure with excellent mass-production capability and reparability can be provided. 

What is claimed is:
 1. A resin compound containing (A) polyamide imide with siloxane bond, (B) acrylic polymer with weight-average molecular weight of 500,000 or more, (C) thermoset resin, and (D) solvent.
 2. A resin compound according to claim 1 , wherein the thermoset resin is a resin that sets by reaction between epoxy resin and phenol compound.
 3. An adhesive film having a resin compound containing (A) polyamide imide with siloxane bond, (B) acrylic polymer with weight-average molecular weight of 500,000 or more, and (C) thermoset resin.
 4. An adhesive film according to claim 3 , wherein the thermoset resin is a resin compound containing epoxy compound, phenol hardener, and imidazole curing accelerator.
 5. An adhesive film according to claim 3 , wherein the glass transition temperature of the acrylic polymer is 0° C. or less.
 6. A metal-clad adhesive film comprising a resin insulation layer, containing (A) polyamide imide with siloxane bond, (B) acrylic polymer with weight-average molecular weight of 500,000 or more, and (C) thermoset resin, and a metal.
 7. A circuit board of a build-up structure on which there is arranged an adhesive film having a resin compound containing (A) polyamide imide with siloxane bond, (B) acrylic polymer with weight-average molecular weight of 500,000 or more, and (C) thermoset resin, said circuit board being electrically connected to external terminals.
 8. An assembly structure in which there is arranged an adhesive film having a resin compound containing (A) polyamide imide with siloxane bond, (B) acrylic polymer with weight-average molecular weight of 500,000 or more, and (C) thermoset resin on a circuit board of a build-up structure, said circuit board being electrically connected to external terminals, and a flip chip is mounted on the circuit board via the external terminals. 